"Osama bin Laden is an iconic figure now, but it doesn't seem he has that much control over actual events or operations."

The website that carried the notice showed a still photograph purportedly from the coming video, with bin Laden addressing the camera, his beard fully black. In his past videos, bin Laden's beard was almost entirely grey with dark streaks.

Bin Laden's beard appears to have been dyed, a popular practice among Arab leaders, Rita Katz, director of the SITE Institute, a Washington-based group that monitors "terrorist" websites, said.

"I think it works for their [al-Qaida's] benefit that he looks young, he looks healthy," she said.

'Huge priority'

The US said that any video message would be closely scrutinised and that efforts to capture the al-Qaeda continued.

"Obviously, it's a huge priority for us to capture and kill bin Laden and bring he and the rest of the leadership to justice," Frances Townsend, the US president's homeland security adviser, told CNN television.

"We're being manipulated every time that they issue a statement, because they're trying to use the media as a way to terrorise us," she said.

Bin Laden is believed to have escaped US efforts to capture him during a December 2001 battle in Afghanistan's Tora Bora mountains.

The al-Qaeda leader is now thought to be hiding in the remote tribal areas along the Pakistan-Afghanistan border.